In a general switching power supply device, when a spike voltage generated by an input winding of a transformer is clamped, a large high-frequency noise generated by a diode included in a clamp circuit affects outside of a power supply device through an input line or through an output line via other windings of the transformer.
Similarly, the large high-frequency noise is generated by a rectification diode used to rectify auxiliary power of the switching power supply device when a flyback voltage is rectified, and affects outside of the power supply device through the input line or through the output line via other windings of the transformer.
The related art will be briefly described below.
FIG. 1 is a circuit diagram of a flyback converter according to the related art. FIG. 2 is a waveform diagram of voltage and currents of components illustrated in FIG. 1.
In FIG. 1, an alternating current (AC) input voltage is rectified and smoothened by a capacitor 11. Through a switching operation of a switching device 12 controlled by a control unit 13, energy transferred from an input winding 231 of a transformer 23 to an output winding 233 thereof is rectified by a diode 16 and smoothened by a capacitor 17 to supply power to output. When the switching device 12 is switched, energy accumulated in a leakage inductance of the input winding 231, which is not coupled to an output winding 233 of the transformer 23, is not delivered to output but is discharged while charging a junction capacitance of the switching device 12, a distributed capacitance of the input winding 231, etc., thereby generating a high spike voltage. To protect the switching device 12, the magnitude of the spike voltage generated by the input winding 231 is limited by a clamp circuit including a diode 18, a capacitor 19, a resistor 20, etc. In FIG. 1, reference numeral 14 represents a diode, reference numeral 15 represents a capacitor, reference numeral 21 represents an input line, and reference numeral 22 represents an output line.
In FIG. 1, while the switching device 12 is at on state, current flows through the input winding 231 and thus magnetic energy is accumulated in the transformer 23. Referring to FIG. 2, a current Ip flowing through the input winding 231 has a maximum value at a time point t1 immediately before the switching device 12 is turned off. When the switching device 12 is turned off at the time point t1 of FIG. 2, the junction capacitance of the switching device 12 or the distributed capacitance of the input winding 231 is charged with the current Ip of the input winding 231 and a voltage Vds between both ends, e.g., a drain and a source, of the switching device 12 is increased due to the magnetic energy accumulated in the transformer 23. When a voltage of the output winding 233 is higher than an output voltage, the diode 16 is turned on and thus the magnetic energy accumulated in the transformer 23 is supplied to the output winding 233. At a time point t2a, when the voltage Vds between the both ends of the switching device 12 becomes greater than or equal to a certain voltage, a forward voltage starts to be applied to the diode 18, and among magnetic energy accumulated in the transformer 23, high magnetic energy accumulated in the leakage inductance, which is not discharged via other windings and remained, forces the diode 18 to be conducted. In this timing, a large amount of high-frequency noise current is generated inside the diode 18. At a time point t2, when the voltage Vds between the both ends of switching device 12 is increased to a level sufficient to makes diode 18 to be conducted, the diode 18 is strongly turned on by the high energy accumulated in the leakage inductance and thus a current of the diode 18 increases while generating a large amount of high-frequency noise current. Accordingly, as shown in a waveform diagram of a diode current Id of FIG. 2C, the diode 18 generates a large amount of high-frequency noise current in a time period from the time point t2a to a time point t2b. A noise voltage is generated from the noise current and affects outside of a power supply device through an input line or through an output line via other windings of the transformer 23.
FIG. 3 illustrates an example of a structure of the transformer 23 included in the flyback converter of FIG. 1.
In FIG. 3, reference numerals 1 to 7 represent numbers allocated to pins of a transformer 23.
The input winding 231 and the output winding 233 are wound around a ferrite core 236 to have a tight magnetic coupling factor. A bias winding 232 is located between the input winding 231 and the output winding 233, and blocks capacitive coupling between input winding 231 and the output winding 233 generated by a switching operation of the switching device 12, thereby reducing a noise potential of a switching frequency component of the output line 22.
In the above-described power supply device of FIG. 1 according to the related art, a high high-frequency noise voltage is generated when a voltage of the switching device 12 is clamped and thus affects outside of the power supply device via the input line or the output line, thereby increasing costs of a line filter, etc.
FIG. 4 illustrates a power supply device according to the related art, in which energy accumulated in a leakage inductance of an input winding 231 of a transformer 23 is absorbed by a capacitor 51 and a resistor 52 to reduce a peak value of a spike voltage.
In FIG. 4, when the peak value of the spike voltage is reduced by absorbing the energy accumulated in the leakage inductance by the capacitor 51 and the resistor 52, an amount of energy to be applied to turn on the diode 18 may be reduced and thus the intensity of high-frequency noise generated by the diode 18 may be significantly reduced. However, since a high voltage is charged at both ends of the capacitor 51, expensive component that can endure high voltage is needed. Furthermore, when a switching device 12 is turned on, a large amount of power is lost as a voltage charged in the capacitor 51 is discharged via the switching device 12.
FIG. 5 is a circuit diagram of another example of a flyback converter according to the related art.
In FIG. 5, when a switching device 12 controlled by a control unit 13 is on, energy is accumulated in a first input winding 251 and a second input winding 252 of a transformer 25. When the switching device 12 is off, an output winding 253 outputs the energy. The output energy is rectified by a diode 16a and smoothened by a capacitor 17 to supply power to output. A flyback voltage generated by the second input winding 252 is rectified by a diode 14b and smoothened by a capacitor 15b and then is supplied as a power supply voltage to a control unit 13. However, after the switching device 12 is turned off, a spike voltage in which an electric potential changes at a high speed is generated from the energy accumulated in a leakage inductance component of the second input winding 252, which is not coupled to the output winding 253. The diode 14b generates a large high-frequency noise voltage when a flyback voltage having spike voltages in which an electric potential changes at a high speed is rectified. The noise voltage affects outside of a power supply device through the input line or through the output line via other windings of the transformer 23, thereby increasing costs of a noise filter for reducing a level of electromagnetic interference (EMI) of the power supply device to a predetermined value or less.
In FIG. 5, a resistor and a capacitor may be connected to both ends of the second input winding 252 to reduce a noise voltage generated by the second input winding 252 but it is difficult to significantly reduce the intensity of high-frequency noise generated by the diode 14b while maintaining a degree of power loss caused by the capacitor to be within a permissible level.